Projecting a vector V orthogonally onto a plane can be thought of
as finding the closest vector in the plane to V. This `closest
vector' always exists; it may be coincident with the original
vector.
Two related routines are VPRJPI, which inverts an orthogonal
projection of a vector onto a plane, and VPROJ, which projects
a vector orthogonally onto another vector.

1) Find the closest point in the ring plane of a planet to a
spacecraft located at POSITN (in body-fixed coordinates).
Suppose the vector NORMAL is normal to the ring plane, and
that ORIGIN, which represents the body center, is in the
ring plane. Then we can make a `plane' with the code
CALL PNV2PL ( ORIGIN, NORMAL, PLANE )
can find the projection by making the call
CALL VPRJP ( POSITN, PLANE, PROJ )